Study On Phase Transformation And Shape Memory Behavior Of Ti-Ni-Nb Alloy

In order to develop Ti-Ni based shape memory alloys and their heat treatment processes with excellent properties,the effects of annealing and aging processes on the phase composition,phase transformation behaviors,microstructure and shape memory behaviors of Ti-50.8Ni-0.1Nb?atomic fraction?alloy were investigated by X-ray diffraction,differential scanning clorimetry,optical microscopy,confocal laser scanning microscopy,transmission electron microscopy and tensile test.The results are as follows.The phase compositions of Ti-50.8Ni-0.1Nb alloy annealed at 350⁷00°C for 0.5h and aged at 300⁶00°C for 1-50h are martensite M?B19',monoclinic?and parent phase A?B2,CsCl?.With increasing annealing temperature,the transformation types of the Ti-50.8Ni-0.1Nb alloy change from A?R?M/M?R?A to A?R?M/M?A to A?M/M?A?A-parent phase B2,CsCl;R-R phase,rhombohedral;M-martensite B19',monoclinic?upon cooling/heating;the R phase transformation temperature and temperature hysteresis decrease,the M phase transformation temperature and the temperature hysteresis increase first and then decrease.The phase transformation type of Ti-50.8Ni-0.1Nb alloy aged at 300⁴00°C for 1²0h is A?R?M/M?R?A,aged at 400°C for 50h and at 500°C for 1⁵0h is A?R?M/M?A,and aged at 300°C for 50h and at 600°C for 1⁵0h is A?M/M?A upon cooling/heating.The R phase transformation temperature of Ti-50.8Ni-0.1Nb alloy aged at 300⁶00°C for 1⁵0h varies in the range of 10³9.8°C,and the M phase transformation temperature varies in the range of-98.3⁸.2?.The temperature hysteresis of R phase transformation in the alloy aged at 400°C for 10h is minimum?3.5°C?,and the temperature hysteresis of M phase transformation in the alloy aged at 600°C for 20h is minimum?23.5°C?.With increasing annealing temperature,the microstructure morphology of the annealed alloy changes from fibrous to equiaxed grain,the recrystallization temperature is about 580°C.With increasing aging temperature,the morphology of Ti₃Ni₄ precipitates in the alloy changes from granular?acicular?lenticular?flaky.The tensile strength of Ti-50.8Ni-0.1Nb alloy annealed at350⁵50°C is higher than that of the alloy annealed at 600⁷00°C,while the percentage elongation of the former is lower than that of the latter.The tensile strength of the alloy aged at 400°C for 20h is maximum?1385 MPa?,and the elongation of the alloy aged at 600°C for20h is maximum?54.2%?.The Ti-50.8Ni-0.1Nb alloy annealed at 400⁵50°C shows shape memory effect,and the alloy annealed at 300°C and 600⁷00°C shows superelasticity.The alloy aged at 300°C for 1⁵0h and at 400°C for 1h shows superelasticity,the alloy aged at400°C for 10²0h and at 500°C for 1⁵0h shows shape memory effect,and the alloy aged at400°C for 5h,at 400°C for 50h,at 600?for 1⁵0h shows shape memory effect and partial superelasticity.With increasing stress-strain cycling number,the platform stress in the stress-strain curve of the annealed and aged Ti-50.8Ni-0.1Nb alloy decreases,the stability of the shape memory effect in the alloy annealed at 400⁵50°C and the superelasticity in the alloy annealed at 600°C and above is well,the stability of the superelastic behavior in the alloy aged at 300 for 1h and at 400 for 1h is well,the stability of the shape memory behavior in the alloy aged at 400°C for 5¹0h is well,and the behavior of the alloy aged at 400°C for20⁵0h and aged at 500?for 1h changes from shape memory to superelasticity.